Bile salts (these are like a detergent that helps in digestion of lipid type stuff)

Two kinds of lipids: Simple and Complex

Complex lipids are defined as being saponifiable; this means they have a releasable fatty acid

Saponification occurs by heating the lipid with KOH. If a fatty acid is released then it is saponifiable and therefore a complex lipid. If not, it is a simple lipid and not saponifiable.

Fatty acids vary by chain length and number of double bonds:

Trivial Name

# Carbons

# Double Bonds

#C.#2xBonds.Pos

Melting Point Temperature

Myristic acid

14

0

14:0

53.9 Degrees C

Palmitic acid

16

0

16:0

63.1

Stearic acid

18

0

18:0

69.9

Oleic acid

18

1

18:1.9

13.4

Linoleic acid

18

2

18:2.9,12

-5

Linolinic acid

18

3

18:3.9,12,15

-11

Aracidonic acid

20

4

20:4.5,8,11,14

Decosahexanoic acid

22

6

22:6.4,7,10,13,16,19

Note there are no odd chains because they are made of acetyl groups (2C).

The number of carbons is not less than 14 because chains that short would be too soluble.

The number of carbons is not more than 22 because then the membrane would be too thick.

As the number of carbons increases, the melting temperature decreases.

As the number of double bonds increases, the melting temperature decreases.

The double bonds keep the lipid from packing together as well and thus cause a lower melting temperature.

Nomenclature:

The Omega carbon is the carbon farthest from the OOC group, so the carbon farthest from the alpha carbon.

Any lipid with its last double bond 9 carbons from the Omega Carbon is part of the Omega-9 family. Likewise, a lipid with its last double bond 6 carbons from the Omega carbon is part of the Omega-6 family of lipids.

Omega-6 encourages cancer and Omega-3 inhibits cancer. And we know that before the Industrial Revolution we ate about a 1:1 ratio but now we eat about a 30:1 ratio of Omega-6:Omega-3 lipids. Hmmm....

Chemistry of Lipids

Almost all carbon-carbon double bonds are cis.

The reason we have trans fats is partial hydrogenation --a process used in food production. The food-makers use this process because it is a cheaper, stable, more controllable method of making stuff with the right consistency properties. This process reduces the double bonds and turns cis double bonds to trans double bonds.

Chain lengths are even.

This is because lipids are made from acetyl groups.

Sometimes we have odd length chains, but rarely and they aren't deadly or anything.

When the number of carbons is less than 10, the lipid will be "liquid" at room temp. When the number of carbons is greater than 12, the lipid will be "solid" at room temp.

Number of double bonds varies between 0 and 6.

Water Solubility

The shorter the chain, the more water soluble.

So we need just the right length to be the right solubility and to form the correct thickness of membrane.

If we put the lipid on a phospholipid, it can change its state at a given temperature.

Fluidity of membrane is how viscous it is.

Free Fatty Acids

These are partially poisonous so they aren't found in significant amounts.

They are like a soap in that they would dissolve membranes.

They are found in aging and diseased states.

Usually free fatty acids are esterified to be made "safe".

Oil vs. Fat

Oils are liquid at room temp.

Fats are solid at room temp.

Both are called triacylglycerol or triglyceride.

Fats and oils are not found in membranes because they are not polar.

Fats and oils are a good storage form of energy. The body can get energy from both the fatty acid and the glycerol.

These hold about 9 kcal/gram, so compared to sugars (4 kcal/gram) they are a very dense form of energy.

There are also diglycerides and monoglycerides.

Diglicerides are found in membranes because they are polar because of the OH group.

Ester bonds hold the acyl groups onto the glyceride.

Ethers can also be used to hold the acyl on, but this is more rare.

C-O-CH2-CH2...

Called an Alkyl ether.

C-O-CH=CH2...

Called an Alkenyl ether.

These have been found in archae bacteria

They were around long ago when the world was "nasty"; they learned to be more efficient and therefore use ether bonds. They even have ether links in their membranes.

We also have ether links in our membranes, though we're aren't sure why.

Posmologens

These lipids have ether at their SN-1 site (which we'll learn about in the next lecture) and an ester bond at their SN-2 site.

We don't know the function of these lipids yet. It is a vestigial molecule.

Phosphoglycerides

AKA phospholipids

These are polar.

It takes 4 dehydrations to make one of these molecules.

There are 7 classes of phospholipids. They are defined by the attached alcohol.

These are made up of the phosphate group.

When making a phospholipid, we have to pull off the protons (so they each have a pKA). However, they are all the same when we look at a plain old phosphate molecule. So, the first dehydration removes the Hydrogen with the highest pKA (10), then the second highest pKa Hydrogen (7.2) is taken, then the lowest pKa hydrogen (2.1).